[R] Testing if a variable is specified within a function & adding TRUE/FALSE options to functions

[Sarah Goslee]

Hi Simon,

On Wed, Jun 29, 2011 at 11:42 AM, Simon Goodman <s.j.goodman at leeds.ac.uk> wrote:
> I have 2 related questions about functions.
>
> 1. I am writing a function to plot data from a time series with the form
>
> myplot<-function(data, d1,d2) {    }
>
> Where d1 and d2 are two dates in a time series. The idea being that if no
> values for d1 and d2 are entered then the function defaults to plotting the
> whole time series, else it plots the data for the interval specified by d1
> and d2.

The usual way to check whether an argument has been specified within a
function is by using missing().


> I am attempting to test if the variable d1 has been inputted by using a
> different function, orginally posted on a R help forum by Brian Ripley.
>
> testObject <- function(object)
> {
>   exists(as.character(substitute(object)))
> }
>
> here testObject(x) returns FALSE if x is not currently present a variable in
> the work space image.
>
> testObject works fine outside my plotting function, but not within it.... it
> always returns FALSE inside the plotting function even when d1 is being
> given by the user.
>
> I get the same result even if the testObject function is defined inside the
> plotting function....
>
> I suspect this may be due to enviroment being searched for d1.... but can't
> find work out how to make it search for d1 within the 'myplot' function - I
> think this can done using 'where' or 'environment' - but the documentation
> on these commands is a little opaque.
>
> 2. For the 'myplot' function I would also like to add a customlegend=TRUE
> (or FALSE) option, which determines if a custom legend is plotted (if not
> inputted it would default to TRUE), but haven't been able to find anything
> on how to specify this kind TRUE/FALSE of option for functions.

I'm not sure what the question is, exactly. You do something like:

myfun <- function(whatever, customlegend=TRUE) {
# name the argument, give it a default value
   plot(whatever)
   if(customlegend) {
      # do the custom legend stuff
   } else {
      # do whatever the other version is
   }
   invisible()
}

It's all a matter of passing arguments to your functions.

If those aren't really what you were asking, then you might need to
restate the questions.

Sarah


-- 
Sarah Goslee
http://www.functionaldiversity.org